Sand Filters

History:


Sand filtration has a long history, the earliest recorded use of sand filtration was around 4000 B.C., in India. During the Roman Republic and during the Roman Empire, municipal water systems used sand filtration to treat the water before it was distributed locally. More recently, in mid 19th Century London, it was noted that in the local area around water pumps where sand water filters had been installed, the prevalence of cholera decreased. This led the British government to install sand filters everywhere water was drawn. In the United States, a rapid sand filter was developed, one that could be quickly cleaned with streams of water.


The Popular Sand Filter:

Within the context of swimming pools, in Maricopa County, the sand filter is by far the most common filter used. There are several reasons for this. One, they are reasonably priced due to a simple design and construction. Two, they are end user friendly. Three, they perform well enough for its intended purpose. Four, they have a long life and five, they are very easy to clean. These advantages really add up, and make the sand filter probably the best all round filter.


Cleaning or Backwashing the Sand Filter:

When the desert dust storms begin in July, the sand filter can quickly remove the dirt, dust, and debris that blows into the pool. Once the filter is full, backwashing really is a snap: turning the pump off, move the backwash valve handle into the backwash position. Then turn the pump back on. During the backwashing process, the flow of water within the sand filter is reversed. The water now enters the filter from the bottom, flowing upward through the sand and lifting the dirt up from the interior and the surface of the sand bed. The now dirty water flows out of the sand filter into a backwash pipe of hose usually into a specially made rock pit or just onto the lawn.


During this process, keep an eye on the water exiting the backwash pipe or hose. When the water
turns from dirty to clear, you can turn the pump off, and move the backwash valve handle back into it's original filtering position. Then start up the pool pump. If you are picky, you can run your pump for a few minutes and then repeat the process and a little more dirt will come out. It doesn't get any easier than this. This procedure can be followed for years without any extra maintenance to the filter.


WARNINGS:

WARNING # 1 : Do not backwash your sand filter too often. A sand filter removes smaller and smaller particles as the filter becomes dirty - in other words it becomes a better filter when it is allowed to dirty up some. Now, here you want to keep an eye on the pressure gauge. Let the pressure build up over time, It is OK! When finally, the circulation to the pool begins to slow down, and the pressure gauge reading gets high, then backwash.

I read an article years ago about an individual who did his masters thesis dissertation on sand filters. He found that a large sand filter when coupled with a high head pump could, (when allowed to get dirty, really plugged), filter particulates down to the size of 1/4th of a micron! He stated in the article that there was a lot of 1/4th of a micron stuff in the filter he studied. This what is called micro-filtration, which will catch algae, bacteria and protozoa. So let's remember not to backwash a sand filter too often.

WARNING # 2 : The sand filter uses a special type of sand media whose grains have many sharp points. It is these points that capture the particles of dust, dirt and debris that pass by. As the years go by, these points get worn down. Then the sand will need to be replaced, but not by any old sand, it has to be sand made in a special way, exclusively for sand filtration. In other words, don't put ordinary sand in the filter, go to the local pool store and buy a few bags.

Situations where the sand grains have worn down and become rounded, will not happen for five, ten or more years. In fact, I have seen thirty year old sand filters that are still functional. You can expect a long service life from this type of filter.

WARNING # 3 : Do not under any circumstances set the sand filter to the backwash position during the vacuuming of the pool or while the suction side cleaner is running - all the smaller debris will pass through the pump strainer basket and be deposited within the sand filters laterals which are at the bottom of the filter, and any dirt exiting the laterals will be deposited at the bottom of the sand bed. This is the reverse of the intended design! It is difficult to get dirt and debris out of these two areas of the filter.

WARNING # 4 : Keep your head and arms away from the top of any filter that is starting to get water from the pump. I have seen from time to time a gate valve installed on the backwash line. Usually it was installed there by someone trying to stop a leaking backwash valve. This is a bad thing to do! I remember trying to backwash a sand filter and because there was a gate valve on the backwash line, in the closed position, as I turned on the pump, the residue air in the filter was compressed by the water, which did not have anywhere to go. Remember that air compresses, water does not. The top lid of the filter blew off about 75 feet into the air. If I had my face or hands in the area of that lid, I would have been injured. So always check to be sure that the backwash pipe is free of gate valves. Always check to be sure that all of the gate valves are open on the lines returning to the pool. Children like to turn valves. If there are any other problems downstream from the filter, the same thing can happen. Remember don't lean or reach over the top of any filter while it is filling with water. In fact, avoid leaning over a pressurized filter at any time.

After turning the pool pump on, always slowly loosen the air relief valve to allow any air trapped in the filter to escape. This valve will usually be located under the pressure gauge. When all the air has been released and water comes out of the valve, close it back up.

The next post will cover the advantages of diatomaceous earth filters.